It is currently possible to transmit audiovisual programs in digital form via all types of broadcasting networks. To avoid the pirating of works broadcast in this manner, the latter are frequently encrypted or scrambled by various well known means.
US 2003/142286 relates to the area of the broadcasting of protected contents by encryption and presents a process for the revocation of receivers within a group. It describes a method of encryption during broadcasting including assignment of a main key to each of the receivers, each main key of which is used to derive two or a plurality of secondary keys. It centers on the management of the generation of secondary keys for the revocation. The revocation of the receiver is performed by indexing the secondary keys derived from the main key to be revoked. A secondary key is then selected that can be derived by the main keys of the authorized receivers, but not by that of the receiver to be revoked. Encryption of the content is performed by the secondary key selected and, thus, all the receivers have access to the content with the exception of the receiver whose main key had been revoked by the secondary key selected. That approach represents a classic encryption solution with the aid of keys and, as a consequence, the entire content remains in the protected stream. Moreover, the content is protected by the same secondary key for a multitude of receivers and, thus, if an ill-intentioned person finds the key and broadcasts it to other members of the group, the content would be accessible by all members of the group.
US 2003/187801 discloses a process for the protection of contents with the aid of encryption in combination with an architecture of the management of rights in which the digital content contains a license that can revoke a receiver. Changing the license for restitution of the content is performed by a modification code associated with the restitution license and coming from a license server. The apparatus of the receiver must have the modification code to have access to the license for restitution of the content and to modify it as a function of the modification code. The restitution license contains keys for decrypting the content and the encryption keys are generated with the aid of classic random functions. That approach represents an encryption solution with the aid of keys and, thus, the entire encrypted content remains in the protected stream. Moreover, such a generation of encryption keys renders the system vulnerable to attacks of the search correlation type between the keys generated for the protection of the content.
WO 98/42098A relates to the protection of a video stream by encryption of the complete stream, then extraction of certain fragments of the encrypted stream. The first stream, that is partial but encrypted, is sent to the subscribers in a free manner whereas the stream of encrypted fragments will be sent to a given user only after obtention of a license after identification. That approach performs an extraction of part of an encrypted stream to obtain to encrypted elements. It has the disadvantage that the first encrypted stream is incomplete and is no longer compatible with the format of the original stream. It can therefore not be viewed or heard by a standard reader (player) and can possibly no longer be transmitted or broadcasted by networks and standard servers.
Furthermore, the fragments extracted from the encrypted stream possibly do not permit the removal of audiovisual elements that would destroy the appearance for a human being. Thus, it is possible to see or hear a part of the audiovisual stream simply by decrypting the first encrypted stream.
WO 02/23315A relates to a system comprising: a first key for decrypting content, a header containing information for playing the content, a module for generating the first key, a module for encrypting the content and removing a first portion of the content and substituting the header for it.
The content can be encrypted with a single key or by segment, each segment being coded with a separate key. The key or keys are then backed up in a database server. During viewing of (listening to) content, the first portion of the content that had been removed is recombined with the decrypted file.
That approach poses the problem of classic content encryption systems with one or several keys, which encryption keys are recorded in a database server; the server is called by the client to recover the keys for decrypting the content at the moment of viewing (listening).
Even if that system is deemed to indicate that a part (the header) is substituted and retained in the server, this is not a substitution for the auto-protection of the content.
EP 1 248 433 discloses a separation of an audiovisual stream into two parts: an insufficient content and complementary content in such a manner as to be able to transmit less information when the user desires to see (listen to) the complete content. The insufficient content is broadcast beforehand or is sent during the broadcast.
That approach does not discuss encryption or protection of insufficient content and/or complementary content. The problem posed by that approach is that it is possible for any user to very easily recover the insufficient content and the complementary content and, therefore, to pirate the system.
Moreover, since the insufficient content is not viewable (audible), it is necessary that the complementary content has a relatively significant size (more than 10%) of the original content, which would have difficulty passing on narrowband networks. The insufficient content is no longer compatible with the format of the original content; the broadcasting and the transmission of the insufficient content can therefore not take place as simply.